Electric starter motors are widely utilized for cranking small gasoline engines such as those utilized in garden tractors, lawn mowers, snow blowers, outboard motors for boats, etc. In such a starter, a pinion drive provides the means for momentarily engaging the engine flywheel in transferring power from the electric starting motor to the internal combustion engine and then disengaging the starter motor from the flywheel once the engine has started to prevent damage to the starter motor. The most common way to facilitate engagement and disengagement of the pinion to the flywheel is to mount the pinion gear to a shaft so that it is rotatably driven by the motor and is axially movable along the shaft. The axial movement allows full engagement of the pinion gear with the flywheel during cranking and complete disengagement once the engine has started. The axial travel of the pinion gear is generally facilitated by one of two means. The pinion gear is either forced along the shaft by a solenoid or by inertia of the pinion gear interacting with the accelerating motor shaft by means of mating helical threads on the pinion gear and on the shaft. Exemplary starter assemblies are disclosed in Kern, U.S. Pat. No. 4,255,982 and McMillan, U.S. Pat. No. 3,690,188 which are incorporated herein by reference. In a typical configuration, the flywheel of an internal combustion engine has gear teeth at its outer periphery and is juxtaposed with a spring biased pinion gear coupled to the output shaft of a starting motor through a torque-limiting friction clutch and a helical spline. When the starting motor is activated and begins to rotate, the inertia of the pinion gear resists rotation and the helical spline causes the pinion gear to translate axially along the starting motor shaft and into engagement with the gear teeth on the flywheel. The engine is thus cranked until the engine speed passes the speed at which it is driven by the starter motor whereupon the helical spline causes the pinion gear to disengage from the flywheel gear teeth. An anti-drift spring operates to urge the pinion gear toward the disengaged position.
As can be appreciated, the starter is subjected to shock and loading stresses as it engages and disengages the engine flywheel. Such stresses are inherent as the motor armature and pinion are rotating as the pinion gear engages the large mass of the flywheel and engine components which are at rest.
Under certain conditions, the engine direction of revolution may suddenly reverse (referred to as "back drive") and, if the operator is re-energizing the starter at that time, the resulting shock is greatly increased and can damage the starter motor, pinion gear and/or flywheel gear. For example, when the starter is engaged and rotating the engine, the engine may fire once and accelerate the flywheel which disengages the pinion. As the engine piston next comes up on the compression stroke, there may not be enough momentum to carry the piston over top dead center and consequently, as the cylinder fires before top dead center, the flywheel reverses. It is also possible that as the piston comes up on the compression stroke, it may not even reach the point where firing occurs, but rather stops under compression. The compressed air in the cylinder will drive the piston back down, thus reversing the direction of the flywheel rotation. If the operator actuates the starter switch as the flywheel is back driven, the pinion which is turning in one direction will engage the back driven flywheel turning in the opposite direction, thus subjecting the starter to great impact and shock that often results in damage to the starter.
In such circumstances, it is desirable to allow slippage in the clutch at a predetermined torque value (referred to as "slip torque") so as not to suddenly reverse drive the armature of the motor and damage the starter.
Accordingly, it is an object of the present invention to provide a friction clutch assembly which limits torque transfer to a predetermined maximum torque value.
Another object of the invention is to provide an electric starter with a clutch assembly having a predetermined slip torque to prevent damage to the starter from engine back drive.
A further object of the invention is to provide such a clutch assembly which utilizes a minimum of components which are economical to manufacture and cost efficient to assemble.
A still further object of the invention is to provide a new and improved method for setting the slip torque of a friction clutch to a predetermined value.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
It has been found that the foregoing and related objectives are attained in an electric starter having an electric motor with a rotatable shaft, a pinion gear for driving a flywheel and a clutch assembly for transmitting torque between the shaft and pinion gear wherein the clutch assembly includes a driving member connected to the shaft for rotational movement and a coupling for frictionally transmitting torque between the driving member and the pinion gear. The coupling has a longitudinal axis and is compressible longitudinally by engagement with the driving member. The clutch assembly and the pinion gear are configured to allow a predetermined maximum compression of the coupling during torque transmission so as to set a predetermined coupling slip torque. In one embodiment, a stop is utilized for engaging the driving member to limit compression of the coupling to the predetermined maximum compression so as to thereby set a predetermined coupling slip torque.
In the method of the present invention, the slip torque of the coupling is determined by limiting the compression of the friction coupling to a predetermined maximum compression which provides the desired slip torque.